1. Field of the Invention
The present invention generally relates to offshore vessel mooring systems that include a turret rotatably mounted within an opening or well within a vessel and connectable to a seabed mooring. More particularly, the invention relates to a method and apparatus for rotatably supporting a mooring turret within a vessel hull.
2. Description of the Prior Art
In recent years, the offshore oil and gas drilling industry has gravitated away from fixed platforms and toward floating storage and production vessels. Under this arrangement, a ship, such as a retired tanker, is moored to a mooring buoy, spider, or similar device connected to the seabed at the location of an undersea well. A riser is connected from the undersea well to the ship for delivering the oil or gas product. In this manner, the ship receives the oil or gas product from the undersea well and acts as a temporary storage facility for the product.
It is desirable in open or unprotected waters to moor the ship to the mooring buoy in such a manner that the ship is free to rotate or swivel about the mooring in a practice known as weathervaning. By this method, the ship is free to move in accordance with the prevailing currents and winds, while still remaining moored to the seabed. This freedom to swivel is commonly accomplished by mounting a cylindrical mooring turret vertically within the ship in such a manner that the turret is able to rotate or swivel about a vertical axis relative to the ship. The turret is commonly moored by one or more mooring lines know as catenaries which extend to the seabed. A mooring buoy, spider, or other connection joint or platform may be used to interface between the catenaries and the bottom of the turret. In addition, one or more oil production risers extend from a wellhead on the seabed into the turret, and the output from the risers is fed into the tanks in the ship for temporary storage.
To enable rotation of the turret relative to the ship, the turret is supported within the turret enclosure by a bearing system. These bearing systems usually include at least one thrust or axial bearing system for supporting axial loads, and at least one radial bearing system for supporting radial loads. Under one conventional arrangement, a thrust bearing system and a first radial bearing system are located near the upper end of the turret, such as on the forecastle of the ship, and a second radial bearing system is located near the bottom of the turret within the turret well. However, it is also known in the art to eliminate the lower radial bearing system to reduce maintenance and alignment problems with the turret, but such an arrangement greatly increases the load and wear on the upper bearing systems. Accordingly, such single-radial-bearing arrangements require an upper bearing system that is durable and compliant.
Also, in the case of smaller ships, turrets having rigid bearing systems have been used successfully to enable the turret to rotate relative to the ship. However, in the case of large turrets, and particularly in heavy seas conditions whereby heaving of the ship may cause vessel hull deflections and substantial loads between the turret and the hull, there is a need for some bearing compliance between the turret and the vessel. Compliant bearing systems used in the past for forming an interface between the turret and the ship include spherical self-aligning bearings, compliant plane bearing systems, and crane-wheel-type bearing systems mounted on springs or rubber pads. However, there is a continuing need for improvement over the conventional turret support systems to achieve a less complex, more efficient, and more reliable support system that maintains compliancy between the turret and the ship.
Under one aspect, the present invention sets forth a novel bearing pad unit for use in the turret support system of the invention. The bearing unit includes a hydrostatic suspension system which enables the bearing unit to accommodate turret fabrication tolerances and also enables the bearing unit to conform to relative movements between the ship and the turret, thereby providing a compliant bearing system. The bearing unit includes one or more bearing plates supported by a hydrostatic load element. The turret includes a stainless steel liner or race which runs directly against the bearing plates of a plurality bearing units. One or more grease ports are provided in each bearing plate to enable the periodic application of lubricant to the interface between the bearing plates and the stainless steel bearing liner of the turret.
In each bearing unit, the hydrostatic load element supports the bearing plate or plates and allows minor realignments of the bearing plates to be made while the bearing plates are under load. The hydrostatic load element includes a bearing pad block upon which the bearing plate or plates are mounted. A cylindrical pedestal engages with a cylindrical cavity located in the bearing block for supporting the bearing block. A pressurized hydraulic fluid is disposed within the cylindrical cavity between the pedestal and the bearing block so that the block is hydrostatically supported. A primary fluid seal and a secondary fluid seal are included at the interface between the pedestal and the bearing unit to prevent leakage of the hydraulic fluid. The primary seal is the main load-bearing seal, and is essentially static in service. The secondary seal is included as a backup should the primary seal fail. Also included in the interface between the pedestal and the bearing block is an annular ring bearing which transmits side loads from the block to the pedestal so as to prevent damage to the seals and to prevent direct contact between the block and the pedestal. In addition, if hydraulic pressure is lost in a bearing unit, the bearing block will be supported by a polymer cushion located on top of the pedestal. The cushion protects the pedestal and the block from high contact stresses by preventing direct metal-to-metal contact between the block and the top of the pedestal if hydraulic pressure is lost.
Pressurized hydraulic fluid may be pumped into the cylindrical cavity to support the bearing block and to put the bearing plates in contact with the turret bearing race surface. A bleed line is included in the bearing block to enable air in the cylindrical cavity to escape when fluid is pumped into the cylindrical cavity. A fluid supply line runs through the pedestal body and the cushion so that the fluid supply line outlet opening is located on the upper end of the pedestal. The fluid supply line is connectable to the pressurized hydraulic fluid circuit, and a plurality of bearing units may be manifolded together by being placed in isolated fluid communication with each other for equalizing the pressure on each bearing unit, thereby providing a self-adjusting feature among a plurality of bearing units.
Accordingly, under an additional aspect, the invention is directed to a system for supporting a turret within a turret well or enclosure. The system includes multiple bearing pad units which serve as thrust and/or radial bearings for supporting the turret. The bearing contact elements are supported hydrostatically so as to compensate for deformations due to fabrication tolerances and vessel hull deflections under load. As a result, the bearing system emulates self-aligning bearings and is able to compensate for axial and angular misalignment. The system allows for monitoring of each bearing unit, automatic lubrication of the bearing surfaces, and in situ replacement of bearing liners should wear or damage occur while the system is in operation.
Under another aspect, the invention sets forth a novel method and apparatus for mounting and operating bearing units for supporting a turret within a turret well in a ship""s hull. Under one embodiment, the thrust and radial bearings are mounted in an equally-spaced manner about the perimeter of the turret bearing surface. The thrust bearing units are all manifolded together so that hydraulic fluid is able to flow between the individual thrust bearing units, but the fluid system is otherwise isolated. Similarly, the radial bearing units are manifolded to other radial bearing units, but otherwise isolated from the fluid circuit so that fluid is able to flow between the radial bearing units, but not to the rest of the fluid circuit. By manifolding a plurality of bearing units together, the pressure applied by the bearing units is self-equalizing so that all the bearing units act in unison to equally support the load, while also allowing some degree of self-alignment and tilting of the load.
In addition, according to another embodiment, the bearing units are mounted in two or more distinct groups, and preferably three groups, with each group being centered 120 degrees apart from adjacent groups of bearing units. The bearing units in each group are manifolded together, so as to act as a single bearing support, but are not manifolded to either of the other two groups of bearing units. This results in the three distinct groups of bearing units behaving as three single bearing pads, thereby providing a self-aligning compliant support, but allowing no tilting of the load. The arrangement of this second embodiment is particularly advantageous in the case of large diameter turrets of, for example, 10 meters diameter and larger.